Fiber-optic torsion sensing using fiber segment interferometry

An interferometric fiber-optic torsion sensing approach based on helical fiber arrangements is presented. The sensors are easy to fabricate and utilize standard single-mode fibers with cleaved ends to provide the interrogated reflection signals, therefore no inscription process is needed. Two sensor designs with sensor diameters of 6 mm, Sensor-1 with a central core and Sensor-2 with two counterrotating helices, were fabricated and compared based on their sensitivities and temperature compensation capabilities. For sensors of active length 76 mm, almost linear readout with noise levels of 3.9 μNmm/√Hz for Sensor-1 and 6.6 μNmm/√Hz for Sensor-2 were achieved over bandwidths of 5 kHz, while suppression of temperature influence by two orders of magnitude was demonstrated for Sensor-2 due to the fully differential principle employed. Because the sensor can be designed with a freely selectable gauge length it is very versatile and could be used in a wide range of structural monitoring tasks, both as stand-alone sensor or integrated within cylindrical structures and shafts.